1. Field of the Invention
The present invention relates to a method and communication device utilized in a wireless communication system, and more particularly, to a method and communication device utilized in the wireless communication system for saving power of a roaming UE corresponding to home cell selection.
2. Description of the Prior Art
A System Architecture Evolution (SAE) system, set forth by 3GPP (3rd Generation Partnership Project) is a new core network architecture for UMTS (Universal Mobile Telecommunications System), addressing cost-efficient deployment and operations for mass-market usage of IP services. The main network evolutions include meeting the targets for a radio-interface of a long term evolution (LTE) system, simplified network architecture, an AII-IP Network, only packet switched means for all services, no circuit switching, and support for mobility and service continuity between heterogeneous access networks, such as between 2G/3G, LTE, non-3GPP access systems.
The SAE core network includes an MME (Mobility Management Entity), an S-GW (Serving Gateway) and a P-GW (PDN gateway). The MME is a control plane entity that manages the attachment to the network, the authentication of the user equipment (UE), and interfaces an E-RAN (evolved radio access network) for the creation of relevant radio bearers.
Functions of the SAE include the Non-Access-Stratum (NAS) functions to be performed by the UE in idle mode, the NAS signalling procedures between the UE and an evolved packet core network (EPC) via an E-UTRAN (evolved UMTS radio access network) including eNBs (evolved Node-Bs), and layer 3 signalling procedures between the UE and the EPC via non-3GPP access networks.
Under NAS functions, home cell deployments are defined as a single eNB used in a house, a building or a small group of cells, e.g. in a campus deployment. Access to home cells is restricted to particular subscribers. In the home cell deployments, a CSG (closed subscriber group) cell as part of the PLMN (public land mobile network) is defined and only suitable for a UE if an identity of the CSG cell is in a whitelist of the UE. That is, the whitelist stored in the UE is a list of CSG cells where the UE is allowed to camp, and the list of CSG cells is usually managed by the PLMN operator.
A roaming UE is a UE traveling from a home PLMN (Public Land Mobile Network) to another PLMN. The PLMN which the UE leaves is called a home network, whereas the PLMN the UE visits is called a visited network. When a UE including a whitelist roams to a visited network, the visited network does not know if the UE has stored the whitelist and thereby does not manages the whitelist. The UE tries to search a CSG cell in the whitelist when camping on a macro cell of the visited network. However, the visited network usually includes none of the CSG cells in the whitelist of the UE which just roams to the visited network. In this situation, the UE periodically performs CSG cell search and no CSG cells can be found. This unnecessary search procedure wastes some degree of UE power.